In the oil and gas producing industry the process of cementing casing into the wellbore of an oil or gas well generally comprises several steps. A string of casing is run in a wellbore to the required depth. Then the well is conditioned by pumping a circulating fluid down through the casing and through the open lower end of the casing and then upwardly through the annular space between the external surface of the casing and the wellbore. This is done to clean the annular space and prepare it to receive the cement. Then a sufficient amount of cement slurry is pumped into the casing to fill the annulus between the casing and the wellbore wall to the desired height. Then a displacement medium, usually a drilling fluid, is pumped into the casing. The cement slurry is normally separated from the circulation fluid and the displacement fluid by rubber plugs. Due to the difference in specific gravity between the circulating fluid and the cement slurry, at first the heavier cement slurry drops inside the casing without having to be pumped by hydrostatic pressure exerted on the displacement fluid. After the height of cement slurry column outside the casing equals the height of the cement slurry column inside the casing, hydrostatic pressure must be exerted on the displacement fluid to force the rest of cement slurry out of the casing into the annulus.
After the desired amount of cement slurry has been pumped into the annulus, it is absolutely necessary to prevent the back flow of cement slurry into the casing until the cement slurry sets and hardens. This back flow is created by the difference in specific gravity of the heavier cement and the generally lighter displacement fluid. One method for preventing the back flow of cement slurry into the casing involves holding constant the hydrostatic pressure on the displacement fluid in the casing until the cement slurry sets and hardens. This method, however, expands the casing and creates nonadherence of the casing to the hardened cement after the hydrostatic pressure in the casing is released and the casing string contracts. Therefore, the preferred method involves placing a check valve in the lower end of the casing string to prevent the back flow of the cement slurry into the casing.
It is sometimes desirable to run the casing with the lower end of the casing fully open to the fluids in the wellbore. This mode of lowering the casing into the wellbore prevents the well fluids from resisting the descent of the casing. The well fluids exert a resistive hydrostatic pressure on the casing when the end of the casing is closed. It is sometimes desirable, however, to completely or partially close the opening in the lower end of the casing so that the well fluids resist the downward movement of the casing. When this is done the resistive forces due to the well fluids acting on the closed end of the casing offset a portion of the weight of the casing string. The resistive forces of the well fluids thereby relieve the derrick of a portion of the load that it bears in supporting the weight of the casing string. Closing the opening in the lower end of the casing also prevents entry into the casing of larger particles suspended in the drilling mud in the bore hole. The presence of larger particles in the casing sometimes causes the opening to be plugged when pumping is commenced.
The presently existing methods for closing or restricting the lower end of the casing string involve the use of devices known as float shoes or float collars. These devices generally comprise tubular members threaded into and made a part of the casing string. If the device is located on the end of the casing string it is generally referred to as a float shoe. If the device is located between two joints of casing it is generally referred to as a float collar.
Both the float shoe and the float collar possess a passageway through the body of the device through which fluid may pass. The passage is equipped with a unidirectional valve which restricts the flow of fluid into the casing from the wellbore but permits the free flow of fluid from the casing into the wellbore. This valve is generally referred to as a check valve. Well known types of check valves include the ball type, the spring-loaded ball type, the plunger type, the flapper type and the multi-flapper type.
Currently existing techniques that involve the use of check valves in float shoes or float collars have certain disadvantages. The check valve in the float shoe or float collar is frequently damaged during the pumping of the circulating fluid through the check valve into the wellbore. Damage to the check valve may prevent the check valve from functioning properly and thereby allow well fluids to enter the casing. If the check valve is damaged it may not function properly during the cementing process. If a damaged check valve permits the leakage of cement back into the well casing, it will become necessary to maintain pressure on the well for an extended period of time during the cementing operation in order to "hold" the cement slurry in place while it sets and hardens. This problem could be avoided if the check valve were not in position within the well casing until after the pumping of the circulation fluid had been completed.
During the pumping of the circulating fluid it is beneficial to have the largest possible bore through the float shoe or float collar in order to have the smallest possible pressure drop across the passage through the float shoe or float collar. Accordingly, it is desirable to run the casing string with no check valve in the casing and then after the casing string has reached the required depth insert the check valve into place within the casing before beginning the cementing process.
If the check valve can be inserted after the casing has been lowered into position within the wellbore, it is possible to choose and utilize a specific type of check valve in response to information concerning the specific well conditions that exist at the desired depth.
During the stage of the cementing operation in which the cement slurry flows out of the casing into the annulus between the casing and the wellbore wall, the speed with which the column of cement slurry drops due to the difference in specific gravity between the cement slurry and the displacement fluid is undesirable. It would be preferable if the flow of cement slurry out of the casing were even, smooth and controlled. This would minimize the chance that the cement slurry would be unevenly distributed throughout the annulus. Thus there is a need for flow control means to control the flow of the cement slurry out of the casing and into the annulus and to prevent the backflow of the cement slurry into the casing after all of the cement slurry has been displaced into the annulus.